In fluid flow apparatus, for example dialysate preparation and supply machines, fluid conditions (e.g., fluid composition, temperature, pressure, and flow rate) are controlled by various control mechanisms (e.g., pumps, heaters, valves, and pressure regulators) and are monitored by fluid condition sensors (e.g., conductivity, pH, level, temperature, pressure, air bubble, and blood sensors). The fluid condition sensors are used both to control the various fluid control mechanisms so as to achieve desired fluid conditions and to monitor the conditions to make sure that they are within safe limits.
In some prior art dialysate preparation and supply machines, the monitoring and control functions are both carried out by a digital processor, and there is on-line verification that the monitoring function is operating correctly. Another prior art dialysate machine approach involves using a digital processor to control the fluid conditions, and analog circuits to monitor the conditions to make sure that safety limits are met.